Perform unused assignment and unused variables lints on MIR. #142390

cjgillot · 2025-06-12T00:34:10Z

The first commit moves detection of uninhabited types from the current liveness pass to MIR building.

In order to keep the same level of diagnostics, I had to instrument MIR a little more:

keep for which original local a guard local is created;
store in the VarBindingForm the list of introducer places and whether this was a shorthand pattern.

I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas.

Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

rustbot · 2025-06-12T00:34:16Z

r? @wesleywiser

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rustbot · 2025-06-12T00:34:17Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

cjgillot · 2025-06-13T01:50:14Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-13T01:50:18Z

⌛ Trying commit 1d8df21 with merge 50ba7f2…

To cancel the try build, run the command @bors2 try cancel.

Perform unused assignment and unused variables lints on MIR. Rebase of #101500 Fixes #51003. The first commit moves detection of uninhabited types from the current liveness pass to MIR building. In order to keep the same level of diagnostics, I had to instrument MIR a little more: - keep for which original local a guard local is created; - store in the `VarBindingForm` the list of introducer places and whether this was a shorthand pattern. I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas. Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

Perform unused assignment and unused variables lints on MIR. Rebase of #101500 Fixes #51003. The first commit moves detection of uninhabited types from the current liveness pass to MIR building. In order to keep the same level of diagnostics, I had to instrument MIR a little more: - keep for which original local a guard local is created; - store in the `VarBindingForm` the list of introducer places and whether this was a shorthand pattern. I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas. Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

bors · 2025-06-13T01:50:47Z

⌛ Trying commit 1d8df21 with merge b0d7157...

rust-bors · 2025-06-13T04:18:05Z

☀️ Try build successful (CI)
Build commit: 50ba7f2 (50ba7f2d41092ac03e7b2d705a93e2e3e56bbf24, parent: 573a01569000d395498a5f98f916d6e5305ac81a)

bors · 2025-06-13T04:18:31Z

☀️ Try build successful - checks-actions
Build commit: b0d7157 (b0d7157996aa3b283f7f26ec09fc81c1612b5fce)

rust-timer · 2025-06-13T06:15:16Z

Finished benchmarking commit (50ba7f2): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.2%, 1.4%]	65
Regressions ❌ (secondary)	0.7%	[0.2%, 2.8%]	103
Improvements ✅ (primary)	-0.5%	[-0.6%, -0.5%]	2
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.3%]	22
All ❌✅ (primary)	0.6%	[-0.6%, 1.4%]	67

Max RSS (memory usage)

Results (primary 0.8%, secondary 0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.0%, 2.9%]	12
Regressions ❌ (secondary)	2.5%	[0.7%, 9.0%]	14
Improvements ✅ (primary)	-4.2%	[-7.0%, -1.4%]	2
Improvements ✅ (secondary)	-6.2%	[-10.9%, -2.3%]	5
All ❌✅ (primary)	0.8%	[-7.0%, 2.9%]	14

Cycles

Results (primary 1.3%, secondary 3.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.9%, 1.7%]	14
Regressions ❌ (secondary)	3.2%	[2.4%, 4.4%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[0.9%, 1.7%]	14

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 754.801s -> 755.907s (0.15%)
Artifact size: 372.21 MiB -> 372.18 MiB (-0.01%)

src/tools/clippy/tests/ui/needless_match.fixed

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that #142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that rust-lang/rust#142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

apiraino · 2025-11-19T11:23:46Z

Nominating this PR for T-compiler discussion to boost signal the Zulip topic about the beta regressions that were reported
#t-compiler > collateral of #142390 on `unused` lints @ 💬

@rustbot label I-compiler-nominated

Perform unused assignment and unused variables lints on MIR. Rebase of rust-lang#101500 Fixes rust-lang#51003. The first commit moves detection of uninhabited types from the current liveness pass to MIR building. In order to keep the same level of diagnostics, I had to instrument MIR a little more: - keep for which original local a guard local is created; - store in the `VarBindingForm` the list of introducer places and whether this was a shorthand pattern. I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas. Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

Pre-compute MIR CFG caches for borrowck and other analyses I was puzzled that rust-lang/rust#142390 introduces additional computations of CFG traversals: borrowck computes them, right? It turns out that borrowck clones the MIR body, so doesn't share its cache with other analyses. This PR: - forces the computation of all caches in `mir_promoted` query; - modifies region renumbering to avoid dropping that cache.

### What Add `#[allow(unreachable_code)]` attribute to the `error_from_error_val` function for wasm targets to suppress a compiler warning introduced in Rust 1.92.0. ### Why I started seeing the following errors when building contracts using the soroban-sdk: ``` warning: unreachable expression --> soroban-sdk/src/env.rs:1770:6 | 1769 | self.env_impl.error_from_error_val(e) | ------------------------------------- any code following this expression is unreachable 1770 | } | ^ unreachable expression | note: this expression has type `Infallible`, which is uninhabited --> soroban-sdk/src/env.rs:1769:9 | 1769 | self.env_impl.error_from_error_val(e) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default ``` It appears Rust 1.92.0 changed how `unreachable_code` warnings are detected. The `error_from_error_val` function returns `Infallible`, an uninhabited type. This is intentional behaviour. The function is designed to never return so the warning is suppressed. I think this is probably the result of rust-lang/rust#142390 which landed in 1.92.0, but that change is quite deep and so I'm not certain of that. In any case, I don't think what the code is doing here is incorrect, just a new warning is highlighting that there's an unreachable condition here, which is expected, so we should allow the condition.

…r=cjgillot Fix suppression of `unused_assignment` in binding of `unused_variable` Unused assignments to an unused variable should trigger only the `unused_variables` lint and not also the `unused_assignments` lint. This was previously implemented by checking whether the span of the assignee was within the span of the binding pattern, however that failed to capture situations was imported from elsewhere (eg from the input tokenstream of a proc-macro that generates the binding pattern). By comparing the span of the assignee to those of the variable introductions instead, a reported stable-to-stable regression is resolved. This fix also impacted some other preexisting tests, which had (undesirably) been triggering both the `unused_variables` and `unused_assignments` lints on the same initializing assignment; those tests have therefore now been updated to expect only the former lint. Fixes #151514 r? cjgillot (as author of reworked liveness testing in #142390)

…r=cjgillot Fix suppression of `unused_assignment` in binding of `unused_variable` Unused assignments to an unused variable should trigger only the `unused_variables` lint and not also the `unused_assignments` lint. This was previously implemented by checking whether the span of the assignee was within the span of the binding pattern, however that failed to capture situations was imported from elsewhere (eg from the input tokenstream of a proc-macro that generates the binding pattern). By comparing the span of the assignee to those of the variable introductions instead, a reported stable-to-stable regression is resolved. This fix also impacted some other preexisting tests, which had (undesirably) been triggering both the `unused_variables` and `unused_assignments` lints on the same initializing assignment; those tests have therefore now been updated to expect only the former lint. Fixes rust-lang/rust#151514 r? cjgillot (as author of reworked liveness testing in rust-lang/rust#142390)

rustbot assigned wesleywiser Jun 12, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 12, 2025

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cjgillot force-pushed the mir-liveness branch from 6eaa375 to 48d9c95 Compare June 12, 2025 00:54

rustbot added A-tidy Area: The tidy tool T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Jun 12, 2025

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cjgillot force-pushed the mir-liveness branch from 48d9c95 to 7104646 Compare June 13, 2025 00:57

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 13, 2025

flip1995 reviewed Jun 13, 2025

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src/tools/clippy/tests/ui/needless_match.fixed Show resolved Hide resolved

cjgillot mentioned this pull request Jun 15, 2025

Pre-compute MIR CFG caches for borrowck and other analyses #142540

Merged

rustbot added the perf-regression-triaged The performance regression has been triaged. label Oct 13, 2025

camsteffen mentioned this pull request Oct 13, 2025

unused_variables and unused_assignments lints do not catch array usage #65467

Open

cjgillot mentioned this pull request Oct 13, 2025

unused_assignments lint has trouble tracking that one match guard observes assignment from another match guard. #138069

Closed

apiraino mentioned this pull request Oct 14, 2025

nightly unused_assignments false positives for miette::Diagnostic fields #147648

Open

Lysxia mentioned this pull request Oct 27, 2025

Update toolchain to 2025-11-13 creusot-rs/creusot#1820

Merged

camsteffen mentioned this pull request Oct 29, 2025

Unreliable warnings about unreachable code downstream of uninhabited expression #107179

Open

TethysSvensson mentioned this pull request Nov 1, 2025

Wrong warning message when using ? on a expression of type Result<Infallible, Infallible> on the 1.92.0-beta.1 #148326

Closed

theemathas mentioned this pull request Nov 15, 2025

unused_assignments not shown for vec overwrite - Rust 1.92.0-beta.3 #148960

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clubby789 mentioned this pull request Nov 17, 2025

unreachable_code lint falsely warns entire function body is unreachable #149042

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workingjubilee mentioned this pull request Nov 18, 2025

unused_variable false positives after diverging (panic!, etc.) #148373

Closed

rustbot added the I-compiler-nominated Nominated for discussion during a compiler team meeting. label Nov 19, 2025

apiraino removed the I-compiler-nominated Nominated for discussion during a compiler team meeting. label Nov 27, 2025

apiraino mentioned this pull request Dec 12, 2025

Regression unused_assignments for variable with underscore prefix captured by closure #149889

Closed

leighmcculloch mentioned this pull request Jan 20, 2026

Allow unreachable_code in wasm error helper stellar/rs-soroban-sdk#1687

Merged

apiraino mentioned this pull request Jan 23, 2026

Unexpected warning about unused assignment in Rust 1.92 and newer #151514

Closed

eggyal mentioned this pull request Jan 23, 2026

Fix suppression of unused_assignment in binding of unused_variable #151556

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nagisa mentioned this pull request Jan 29, 2026

clippy: fix or silence rust 1.92 signaled warnings anza-xyz/agave#10245

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Perform unused assignment and unused variables lints on MIR. #142390

Perform unused assignment and unused variables lints on MIR. #142390

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Overall result: ❌✅ regressions and improvements - please read the text below

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